From Dom Guéranger's The Liturgical Year.

TWO bright stars appear this day on the ecclesiastical cycle, proclaiming the glory of our Jesus, the Conqueror of death. Again they are two pontiffs, and martyr pontiffs. Cletus leads us to the very commencement of the Church, for he was a disciple of Peter, and his second successor in the See of Rome. Marcellinus was a witness of the great persecution under Diocletian; he governed the Church on the eve of her triumph. Let us honour these two fathers of Christendom, who laid down their lives in its defence; and let us offer their merits to Jesus, who supported them by his grace, and cheered them with the hope that one day they would share in his Resurrection.

The following is the account given of St Cletus by the Liturgy:

Cletus Romanus, patre Emiliano, de regione quinta, e vico Patricio, imperatoribus Vespasiano et Tito, Ecclesiam gubernavit. Is ex præcepto Principis Apostolorum, in Urbe viginti quinque presbyteros ordinavit. Primus in litteris verbis illis usus est: Salutem et Apostolicam benedictionem. Qui Ecclesia optime constituta, cum ei præfuisset annos duodecimo menses septem, dies duos, Domitiano imperatore, secunda post Neronem persecutione, martyrio corona tus est, et in Vaticano juxta corpus beati Petri sepultus.
Cletus, the son of Emilianus, was a Roman of the fifth region and of the Patrician street. He governed the Church during the reigns of the emperors Vespasian and Titus. Agreeably to the order given him by the Prince of the Apostles, he established five and twenty priests in the City. He was the first who in his letters used the words: ' Health and Apostolic benediction.' Having put the Church into admirable order, and having governed it twelve years, seven months, and two days, he was crowned with martyrdom under the emperor Domitian, in the second persecution following that of Nero, and was buried in the Vatican, near the body of St Peter.

The Life of St Marcellinus is thus given in the Breviary:

Marcellinus Romanus, ab anno ducentesimo nonagesimo sexto ad annum trecentesimum quartum in immani imperatoris Diocletiani persecutione Ecclesiae præfuit. Multas pertulit angustias ob improbam eomm severitatem qui eum redarguebant de nimia indulgentia erga lapsos in idololatriam, quæque causa fuit ut per calumniam infamatus fuerit, quasi thus idolis adhibuisset. Veruna hic beatus Pontifex in confessione fidei una cum tribus aliis Christianis, Claudio, Cyrino et Antonino, capite plexus est. Quorum projecta corpora cum triginta sex dies jussu imperatoris sepultura caruissent, beatus Marcellus a sancto Petro in somnis admonitus, Presbyteris et Diaconis, hymnis ac luminibus adhibitis, honorifice sepelienda curavit in cœmeterio Priscillæ, via Salaria. Rexit Ecclesiam annos septem, menses undecim, dies viginti tres: quo tempore fecit Ordinationes duas mense decembri, quibus creavit presbyteros quatuor, episcopos per diversa loca quinque.
Marcellinus, a Roman by birth, ruled over the Church from the year two hundred and ninety six to the year three hundred and four, during the terrible persecution of Diocletian. He had much to suffer from the impious severity of those who reproached him with showing too much indulgence towards such as had relapsed into idolatry, whence ensued a calumnious report of his having offered incense to idols. But in truth, this blessed pontiff was beheaded for the confession of the faith, together with three other Christians, Claudius, Cyrinus, and Antoninus. Their bodies, by the emperor’s order, were left six and thirty days without burial, after which the blessed Marcellus, in consequence of his receiving, whilst asleep, an admonition from St Peter, had them buried in the Cemetery of Priscilla, on the Salarian Way; at which burial were present many priests and deacons, who, with torches in their hands, sang hymns in honour of the martyrs. Marcellinus governed the Church seven years, eleven months, and twenty three days. During this period he held two ordinations in December, at which four were made priests, and five bishops for divers places.

Pray for us, O holy Pontiffs, and look with fatherly love upon the Church on earth, which was so violently persecuted in your times, and at the present day is far from enjoying peace. The worship of idols is revived; and though they be not of stone or metal, yet they that adore them are as determined to propagate their worship as were the pagans of former days to make all men idolaters. The gods and goddesses now in favour are called Liberty, Progress, and Modern Civilization. Every measure is resorted to, in order to impose these new divinities upon the world; they that refuse to adore them are persecuted; governments are secularized, that is, unchristianized; the education of youth is made independent of all moral teaching; the religious element is rejected from social life as an intrusion: and all this is done with such a show of reasonableness that thousands of well-minded Christians are led to be its advocates, timid perhaps and partial, but still its advocates. Preserve us, O holy martyrs! from being the dupes of this artful impiety. It was not in vain that our Jesus suffered death, and rose again from the grave. Surely after this he deserves to be what he is—King of the whole earth, under whose power are all creatures. It is in order to obey him that we wish no other liberty save that which he has based upon the Gospel; no other progress save that which follows the path he has marked out; no other civilization save that which results from the fulfilment of the duties to our fellow-men, which he has established. It is he that created human nature, and gave it its laws; it is he that redeemed it, and restored to it its lost rights. Him alone, then, do we adore. O holy martyrs! pray that we may never become the dupes or slaves of the theories of human pride, even if they that frame or uphold them should have power to make us suffer or die for our resistance.